Use www.rfc-editor.org for RFC text.

We use the following site for that now:

* https://tools.ietf.org/ or http
* https://datatracker.ietf.org or http

Today, IETF said the official site of RFC is www.rfc-editor.org.

FYI: https://authors.ietf.org/en/references-in-rfcxml

I replaced them to www.rfc-editor.org.

lib/webrick/httprequest.rb

@@ -402,7 +402,7 @@ def fixup() # :nodoc:
     # This method provides the metavariables defined by the revision 3
     # of "The WWW Common Gateway Interface Version 1.1"
     # To browse the current document of CGI Version 1.1, see below:
-    # http://tools.ietf.org/html/rfc3875
+    # https://www.rfc-editor.org/rfc/rfc3875
 
     def meta_vars
       meta = Hash.new